Transmitter



April 28, 1936.

w. T. DITCHAM TRANSMITTER Filed Junne 29 INVENTOR 4jaten'tecl pr. 1:93

UNITED STATES f TRANSMITTER William Theodore' Ditcham, Chelmsford, England, assigner to Radio Corporation of America, a corporation of Delaware Application June 29, 1933, Serial No. 678,138 s In Great Britain July Z2, 1932 4 claims. (ci. 179-171) This invention relates to radio and other modulated carrierwave transmitters and more particularly to such transmitters of the kind in which a source of Vhigh frequency oscillations supplies carrier wave oscillations to a succeeding high frequency amplifier and in which modulation is effected at this, .the driven amplifier, by the well known series modulation or constant voltage control method. A transmitter in which modulation is effected by the series modulation method is one wherein an amplifier tube, the driven tube, to which the modulating potentials are applied is connected` in series with a modulating tube, the said modulating tube having its cathode at earth potential and its anode connected to the cathode of the said amplifier tube.

In transmitters of the kind to which the present invention relates it will be seen that the supply anode voltage is divided between the amplifier to which drive is supplied-this amplifier will be hereinafter termed the modulated amplifier-and the modulating tube since these two tubes are in series. It follows therefore that the total anode supply voltage must of necessity be high and may be too high in certain casesy for convenient and efcient application'to the high frequency driving stage or stages preceding the modulated amplifier. In such cases therefore it has hitherto been found necessary either to provide a separate supply voltage for the driving stage or stages orV alternativelyrto'use the same source of voltage both for the driving stage and the modulated amplifier, but to reduce the actual voltage applied to the former by inserting a suitable volts dropping resistance in the supply lead. This course involves waste of energy.V

The present invention has for its object to avoid these practical difficulties.

According to one feature of this invention the tube 'driving the modulated amplifier in a transmitter of the kind referred to is in effect connected across the anode and cathode of the said modulated amplifier so that both these valves are subjected to the same supply voltage and are both simultaneously modulated when the grid voltage of the modulating tube is varied. Alternatively a separate auxiliary modulating tube may be provided for the driving stage and means may be provided for applying the modulating potentials simultaneously and in phase both to the main modulating tube associated with the modulator amplifier and the auxiliary modulating tube associated with the drive.

Where the drive tube preceding the modulated amplifier has its anode and cathode connected across the anode and cathode of the said modulated amplifier it will probably be found desirable to provide high frequency chokes to eliminate reactions between these two tubes and if desired also low frequency decoupling means of 5 any form well known per se and comprising impedances and capacities may be included for a similar purpose. 1

' The cathode of both the modulated amplifier and the drive preceding the said amplifier may 10 be heated from a common source of voltage which may be any suitably insulated source of direct current or alternating current voltage. A

Although in the preceding description the modulating amplifier and the driving stages have 15 been referred to as amplifier and the driving stages have been referred to as though each were a single tube, obviously all or any of these tubes maybe constituted by a plurality of valves in parallel or in phase opposition in any other 20 known arrangement.

It will be appreciated that in a transmitter comprising a modulated amplifier and a driving stage simultaneously modulated as hereinbefore set forth, the amount of power from the driving 25 stage only requires to be sufficient to deal with the carrier power to be taken from the modulated amplier and need not be sumcient to deal with the"peak modulated power as would be the case were the driving stage not simultane- 30 ously modulated.

Two embodiments of the inventionV have been illustrated diagrammatically in the accompanying drawing in which Figure 1 illustrates my novel modulation system in which the modulating 35 tube impedance is connected in series with the output impedance of the amplifier tubes with respect to the high potential source; while, Figurev 2 illustrates the modification of the arrangementof Figure 1, in which two modulating tubes 40 had their anode to cathode impedances connected in series with the anode to cathode impedances of two of the carrier frequency relay tubes.

Referring to the drawing, V1 is the driving tube, V2 the modulated amplifier, and V3 the 45 modulating tube. As will be seen, the tube V2, Whose self capacity effect is neutralized by a neutralizing condenser NC, has its grid coupled by radio frequency circuits to the plate of the tube V1 and its cathode to the cathode thereof. 50 The grid V1 is energized by carrier waves to be modulated. The anode to cathode impedance of tube V3, to the grid of which modulating potentials is applied via the secondary winding of Y transformer T3, is effectively in series between 55 the earth point and the cathode point of the tubes V1 and V2. The primary winding of transformer T3 is connected with a source of modulating potentials. The positive and negative supply terminals are represented at HT+ and HT- respectively while T1 and T2 are lament heating transformers for the tubes V1 and V2. High frequency choking inductances RFC are inserted between the anodes of tubes V1 and V2 and the source of high potential to prevent the carrier waves being modulated from reaching the high potential source. Radio frequency choking inductances RFC are also inserted between the anode of tube V3 and the cathodes of -tubes V1 and V2 in Figure 1 and between the anodes of tubes V3 and V4 and the cathodes of tubes V2 and V1 respectively in Figure 2 to prevent carrier frequency oscillations from reaching the modulating tubes and source.

In the modications shown in Figure 2 the connection between the anode of V3 and the point on the cathode heating circuit of V1 is removed and a second tube Vi has its anode connected as shown to a point on the heating circuit of V1, its cathode connected to ground and to the Y negative terminal of the high potential source and its control grid connected in parallel with the control grid of V3.

In operation in the modification oi Figure 1 modulating potentials are applied to the control grid of tube V3 to modulate the anode to cathode impedance thereof at signal frequency. The anode to cathode impedance of tube V3 determines the conductivity of the circuits including the anode to cathode impedance of the relays or amplifiers V1 and V2. In this manner the carrier wave relayed or amplified in the driving stage V1 and the driven stage V2 is modulated at signal frequency.

In the modications shown in Figure 2 modulating potentials are applied in phase tothe control grids of tubes V3 and V4 to control in likeV senses the impedances between the anodes and cathodes of said tubes. The anode to cathode impedance of tube V3 is in series with the anode A to cathode impedance of V2, While the anode to cathode impedance of V4 is in series with the anode to cathode of impedance V1. In this manner the carrier wave relayed or amplified in tubes V1 and V2 is modulated at signal frequency.

The invention is obviously applicable generally for the transmission of speech or music or for the transmission of telegraphy, facsimile signals and so forth.

Having thus described my invention and the operation thereof, what I claim is:

1. In a modulating system, a plurality of thermionic tubes each having a control grid, an anode and a cathode, a circuit for applying carrier waves to be modulated to the control grid and cathode of the first of said tubes, circuits coupling the anode and cathode of said first tube to the control grid and cathode of another of said tubes, a modulator tube having anode, cathode and control grid electrodes, a circuit for applying modulating potentials between the control grid and cathode of said modulator tube, a source of direct current potential, radio frequency choke inductances connecting the anode of said modulator tube to the cathodes of said first named tubes, a connection between the cathode of said modulator tube and the negative terminal of said direct current source, and radio frequency choke inductances connecting the anodes of said first named' tubes to the positive terminal of said source.

2. A modulating system comprising, a pair of thermionic tubes each having anode, cathode and control electrodes, radio frequency circuits connecting said tubes in cascade and to a source of carrier wave energy, a pair of modulating tubes each having a control grid, a cathode and an anode, a circuit for applying modulating potentials in phase to the control grids of said modulating tubes from a source of direct current potential, and circuits connecting the anode to cathode impedance of each of said direct current tubes in series with the anode to cathode impedance of a different one of said first named tubes and with said source of modulating potential.

3. A modulating system comprising, a pair of therrnionic tubes each having an anode, a cathode, and a control electrode, radio frequency circuits connecting said tubes in cascade and to a source of carrier wave energy, a modulating tube having a control grid, a cathode. and an anode, a source of modulating potentials connected to the control grid and cathode of said modulating tube, separate conductive reactances connecting the anode of said modulating tube to the cathode of each of the tubes of said pair of thermionic tubes, and a source of direct current potential having a point thereon connected to the anodes of the tubes of said pair of tubes and a second point thereon connected to the cathode of said modulating tube.

4. A modulating system comprising, a pair of thermionic tubes each having anode cathode and control electrodes, radio frequency circuits connecting said tubes in cascade and to a source of carrier Wave energy, a pair of modulating tubes each having a control grid, a cathode and an anode, a circuit connected with the control grids of said modulating tubes for applying thereto modulating potentials, a source of direct current potential, and circuits including radio frequency choking means connecting the anode to cathode impedance of each of said modulating tubes in series With the anode to cathode impedance of a different one of said first named tubes and with said source of direct current potential.

WILLIAM THEODORE DITCHAM. 

